Echo in a communication system, such as the one shown in FIG. 1, is commonly characterized as the return of a part of a transmitted signal from an end user back to the originator of the transmitted signal after a delay period. As is known in the art, a near end user transmits an uplink signal to a far end user. Conversely, the near end user receives a downlink signal from the far end user. For example, echo at the near end occurs when the near end user originates an uplink signal on an uplink path, and a part of the transmitted signal is reflected at the far end as an echo signal on a downlink path back to the near end. Echo at the far end occurs when the far end user originates a downlink signal on the downlink path, and a part of the transmitted signal is reflected at the near end as an echo signal on the uplink path back to the far end. The reflection of the transmitted signal may occur due to a number of reasons, such as an impedance mismatch in a four/two wire hybrid at the far end or feedback due to acoustic coupling in a telephone, wireless device or hands-free speaker phone. An echo signal corresponding to the delayed transmitted signal is perceived as annoying to the near end user and, in some cases, can result in an unstable condition known as “howling.”
Echo cancellers are desirable at any echo generating source at both the near end and at the far end in an attempt to eliminate or reduce the transmission of echo signals. Echo cancellers may be employed in wireless devices, such as personal data assistants (PDAs), cellular phones, two-way radios, car-kits for cellular telephones, car phones and other suitable devices that can move throughout a geographic area. Additionally, echo cancellers may be employed in wireline devices, such as hands-free speaker phones, video and audio conference phones and telephones otherwise commonly referred to in the telecommunications industry as plain old telephone system (POTS) devices. Hands-free speaker phones typically include a microphone to produce the uplink signal, a speaker to acoustically produce the downlink signal, an echo canceller to cancel the echo signal and a telephone circuit.
Echo cancellers, such as the one shown in FIG. 2, attempt to cancel the echo signals produced at the near end when the far end is transmitting by generating echo estimation data corresponding to a portion of an amplified downlink audio signal traveling through the acoustic coupling channel between the speaker and the microphone. There is shown in FIG. 2 a simplified block diagram of an equipment configuration for one terminal of a communication link which includes a near end hybrid, operative in accordance with some embodiments of the present invention. The communication link may have a near-end comprising a telephone or other voice communication device 2, a four-to-two wire hybrid circuit 3, and an echo canceller circuit 4 including a filter 5 and a Non-Linear Processor (“NLP”) 6. A far-end connected to communication network 23, can be similarly configured but is not illustrated in FIG. 2. During a conversation between a near-end user and a far-end user, the far end signal, x, which contains both the far-end user's speech and incidental background noise, may enter the near-end as signal x at node 9.
Echo cancellers, such as the ones shown in FIGS. 1 and 2 may model the acoustic coupling channel and in response generates the echo estimation data through the use of an echo canceller adaptive filter. Echo cancellers may use an adaptive filter employing modeling techniques using for example a Least Mean Squared (LMS) finite impulse response (FIR) filter having a set of weighting coefficients to model the acoustic coupling channel or other similar modeling techniques known in the art. An echo canceller's adaptive filter may attempt to subtract the echo estimation data from pre-echo canceller uplink data received by the microphone in order to produce post-echo canceller uplink data. The post-echo canceller uplink data may be used by the echo canceller adaptive filter to dynamically update the weighting coefficients of the finite impulse response filter.
FIG. 1 illustrates a exemplary communication link between two telephones or other voice communication devices 24 and 25. The link is comprised of a near-end, a far-end, and a communication network 23 that interconnects the near-end and far-end. The near-end may have a user voice communication device 24, a hybrid circuit 26, and an echo canceller circuit 4A. Similarly, the far-end 22 may have a user voice communication device 25, a hybrid circuit 27, and an echo canceller circuit 4B. Both echo canceller circuits, 28 and 29, may be analogous to the echo canceller 4 shown in FIG. 2.
Far-end signal power, X, is received by the near-end. Signal Y is the coupled echo signal from the far-end signal as well as the near-end signal produced by communication device 24. This near-end signal contains both the speech of the near-end telephone user and the background noise of the user's environment. Together, the near-end signal and far-end echo signal are represented by Y.
The far-end signal is provided to the four-to-two wire hybrid circuit 3 (FIG. 2) and then to near-end communication device 2. Due to the unavoidable non-linearity present in the hybrid circuit 3, some portion of the far-end signal power is coupled onto the output 7 of the hybrid circuit 3 as an echo. A composite signal y exists at node 7 containing the echo signal and the combined speech of the near-end user and any incidental background noise from the near-end user's environment. A filter having a filter length period selected and designed to be longer than the hybrid dispersion time may be used prior to power level measurements at 7 to allow the echo canceller 4 to operate properly.
Echo canceller 4 may synthesize the expected value e of the echo signal in adaptive filter 5, and subtracts this value at 10 from the composite signal y existing at node 7. The resulting difference signal, d, existing at node 14, is intended to contain only the near-end signal s originating from telephone 2. Difference signal, d, may be provided to the far-end telephone through the communications network 23.
Methods of measuring the echo return loss typically measure a signal at node 9, where the signal power from the far-end would normally exist. A measurement of the signal power, x, at node 9 is made. Additionally, the power level of the composite signal y, comprised of the coupled echo signal and any signal s generated by the near-end telephone 2, is measured at node 7. The measurement can be made when little-to-no signal is being generated at near end telephone 2. Assuming the signal power of any signal generated by the near-end telephone is very small in comparison to the coupled echo signal power, the ratio of the measured test signal power x to the measured power level y provides an estimate of the echo return loss (ERL) for the near-end 8. The magnitude of echo return loss is usually measured as a difference in dB between signal x and signal y. Echo return loss may be measured dynamically during the course of a telephone conversation.
Echo is an important factor in communications which include a hybrid between a four wire communication network 23 and the end terminals 24 and 25 as illustrated in FIG. 1. When echo is present, it is preferable to eliminate the echo. To eliminate the echo, the magnitude of the echo must be determined. One way of determining the magnitude of the echo is through echo return loss (ERL) estimation. A high echo return loss means that there is very little echo because most of the energy from the far end has been lost when the near end signal combined with echo is measured.
A typical echo canceller, as illustrated in FIG. 1, includes an adaptive finite impulse filter FIR 5. Under the control of an adaptation algorithm, FIR filter 5 models the impulse response of the echo path. A non-linear processor (NLP) 6 can be used to remove residual echo that may remain after linear processing of the input signal. The echo canceller may also typically include a double talk detector 11. Double talk occurs when both far end and near end speech are present at the same time. A double talk detector 11 can also be used to control and inhibit the adaptation process of the FIR 5 and/or the NLP 6 when double talk is present and it may be undesirable to cancel or suppress echo because double talk will be suppressed.
In the echo canceller, the signal y is the perceived near end signal. Signal y is a combination of the actual near end signal s and the echo from the far end signal x which comes through hybrid 3. The output signal d is the signal y less the echo estimate e generated by the adaptive filter 5. The adaptive filter 5 is programmed to generate an output signal e that is as close to the echo as possible so that the echo is largely cancelled by the echo estimate e and the difference signal d closely resembles the generated near end signal s. The NLP 6 controls the amount of signal d that is transmitted to the far end. When there is no near end signal s, or a large echo over riding near end signal is present, NLP 6 can provide comfort noise to the far end instead of near end signal so as to prevent any possible uncancelled echo from being transmitted. When a valid s exists, NLP opens so as to let the far end hear the signal. False detection of a lack of near end signal s can cause clipping of speech and failure to detect echo can result in echo leak through the NLP. The NLP as an on/off switch can result in abrupt audible changes which are undesirable in speech communications.
There is a need in the field of communication for improved methods and circuits for echo cancellation.